In order to increase the capacity of an optical disc, a laser beam with a short wavelength and an object lens with a large numerical aperture are used for writing and reading information. Thus, a laser spot condensed on a recording layer of the optical disc can be made small, and recording density can be improved.
Because a laser beam flux penetrates a protection layer of the optical disc and reaches the recording layer, a focal length slightly differs between the inside and the outside of the laser beam flux due to change in the thickness of the protection layer, and therefore a spherical aberration occurs. The spherical aberration can be corrected by adjusting the laser beam flux incident to an object lens to a weakly divergent beam and a weakly convergent beam. Adjustment of the laser beam flux is achieved by moving the position of a collimator lens inside the optical pickup in the optical axis direction.
The collimator lens can be moved by a screw feed mechanism converting rotational motion of a small motor to translation motion. The screw feed mechanism is constructed of a lead screw, a rack gear (or a nut) fitted into a thread groove thereof, and a guide shaft. By rotation of the small motor, the rack gear (or the nut) is pressed by the thread groove of the lead screw and moves in the motor shaft direction. The collimator lens is moved by keeping a lens holder mounted with the collimator lens be pressed to the rack gear (or the nut).
As a related art of the present field of the invention, there is Japanese Patent Laid-open No. 2005-216344 (PTL 1) for example.
In PTL 1, it is disclosed that a rack body is configured that teeth attached to the surface of a board abut upon the surface of a lead screw shaft, the rack body is moved by rotation of the lead screw shaft, and the lens is moved.
Thus, PTL 1 discloses a screw feed mechanism using a rack gear as a moving means for an optical pickup mounted on an optical disc drive.
Further, similarly to PTL 1, Japanese Patent Laid-open No. 2009-26410 (PTL 2) also discloses, as a moving mechanism for the collimator lens, a structure in which meshing teeth attached to the surface of a rack member move along the thread ridge of a lead screw.